I am putting an offer on a house in san francisco and part of the “investment” opportunity is to finish the basement, which is completely unfinished at the moment.
The ceilings would need to be about 2 feet higher and I’m wondering what a very rough ball park estimate is for excavation in San Francisco.
Let’s say its about 1000 sq ft of living space and 2 ft excavation.
Thank you so much for any advice you all can give. It’s such a tough market and I’d hate to get into something which was too expensive to remodel.
The short answer is “probably somewhere between $200K and $500K, but possibly more if the project is complicated”. That amount would get you an unfinished space with concrete walls and slab floor.
The digging part isn’t significant - it’s that you would almost certainly be replacing the entire foundation, including the walls, with significant additional work to connect the new foundation to the structure above, plumbing work (will water drain from the new, lower floor?), managing rain water, etc. Working in S.F. in close proximity to other structures requires great care and is still risky. All the while the house needs to be supported in earthquake-safe fashion. It’s considerably more expensive than it would be to build a new foundation of the same size on an empty lot.
It’s tempting to think of deepening a basement as consisting of excavating lower and then just adding more to the bottom of the existing walls, but foundations don’t work that way - retaining walls (most full basement walls are retaining walls) need to be single structural units from top to bottom.
Costs and feasibility would also depend on soil conditions - depending on the neighborhood, San Francisco houses may rest on sand, mud, rock, clay, or old landfill.
Do you have an idea about the soil conditions, ledge, stone, fill, top soil, gravel? Once you purchase the home, you can excavate a couple test pits outside close to the foundation. That would give an indication, and any estimate would be conditional on the excavation results. Then, you can excavate a small section at a time, and underpin the foundation a section at a time, skip a section and come back later to do it. Keep in mind if you have mechanicals in the basement, these need to be raised or removed and reset in place after work is done. Excavated materials can be placed outside to level some areas or removed in dumpsters.
There is not enough info on what exactly is entailed for the project, as it is affected by the way the house is constructed and the topography on which it is situated.
By the sound of things, you have a crawlspace under the house that you want to turn into a livable space. If that is the case, the excavation is the least of your worries; there is a lot more to it that just that.
You will probably need new foundation for the house (or most of it).
You will need shoring and temporary support to accommodate that process.
You will need design and architectural drawings, structural engineering calculations, and a soil engineering / soil report / geo-technical report at the very least for your permit.
This could cost $250K - $500K depending on what is required.
As an engineering contractor, I would help the owner identify the areas of savings and we could tame the cost down by designing what would give the best effect for the least cost.
